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EuroWire – September 2007
100
english
The components consist of the following:
Filler/Bedding
: Extruded halogen free
non-hydroscopic polymeric bedding or
optional non-hydroscopic filler yarns.
Polymeric Protection
:
Impact resistant,
shock absorbing extruded polymer capable
of reducing risk of permanent deformation
and damage to the underlying core.
Metallic
Shield
:
Copolymer
coated
aluminium tape longitudinally applied
with sealed overlap.
Polymeric Layer
: Extruded layer bonded
to the underlying metallic shield barrier.
This combination is resistant to aggressive
chemicals such as hydrocarbons, solvents,
acids, bases and moisture.
Sheath
: Extruded overall low temperature,
flame and sunlight resistant polyvinyl
chloride or low smoke halogen free jacket.
3. Performance of
polymeric armour
3.1 Impact Performance
Comparative impact testingwas conducted
in apparatus designed in accordance with
EDF Specification HN 33-S-52
[5]
. The test
was conducted at different energy impact
levels and employed an impact tool of a
90° V shaped wedge with 80mils (2mm)
radius tip.
After a single impact at the specified
energy level, the thicknesses of various
layers and local damage on the extruded
insulation shield – by means of an optical
laser system – was measured with an
electronic digital caliber.
Testing continued on the three conductor
2/0 AWG 15kV rated cables employing
polymeric
armour
and
continuous
corrugated
aluminium
armour
to
determine the magnitude of impact on
each design that resulted in the same level
of damage on the ethylene propylene
rubber (EPR) insulated conductor. This
was determined to be an impact level
of 200 joules for the polymeric armour
design as compared to 140 joules for the
continuous and corrugated aluminium
metal clad cable design.
Further testing on three conductor
350 kcm 15 kV rated cables found the
impact magnitude that resulted in the
equivalent level damage on the insulation
shield of the EPR insulated conductor was
250 joules for the polymeric armour and
200 joules for the continuous & corrugated
aluminium metal clad cable design.
Impact testing was also conducted on
typical 600 volts rated control cables.
The typical cable configuration of nine
conductors #12 AWG conductor cables
was employed. The testing apparatus
and impact tool design were identical
as employed for impact testing of the
15kV power cable sizes.
This technology has also been adopted
in communications and optical fibre
cable designs for terrestrial and aerial
applications replacing metallic armour/
sheaths as successfully demonstrated in
power and control cables. The severity
of an 80 joules impact can easily be
seen in
Figure 6
. At this impact level the
measured damage on the insulated core
is two times greater on the continuous
corrugated aluminium armour than
the
polymeric
armour.
This
can
be seen in
Figure 8
where exposure of
the #12 AWG conductors through the
insulation was observed.
The criticality of such exposure is the
potential to lose circuit integrity via phase-
to-phase or phase-to-armour and poten-
tially short circuiting and loss of power to
critical equipment and instrumentation
in an industrial or commercial facility. The
insulation within the core of the polymeric
armour cable, while exhibiting some
damage, is not in jeopardy of a phase-to-
phase short circuit.
3.2 Sidewall Bearing
Pressure Performance
Sidewall Bearing Pressure (SWBP) develops
when a cable is pulled around a bend
under pulling tension. It is the vector sum
of the sidewall pressure due to tension in
the conductor acting horizontally, and the
weight of the conductor acting vertically.
SWBP should always be calculated for
that conductor that presses hardest on
the inside bend of the curvature, ie, pipe,
conduct, wheel, etc.
In most cases sidewall bearing pressure
limits for power cable have been demon-
strated to be quite satisfactory via 30 to
50 years of historical data. These were
initially based upon theoretical methods
and consequently, safety factors were
incorporated in the equations. Currently,
North American standards do not define
a test protocol for determination of
sidewall bearing pressure.
Polymeric armour
Continuous corrugated
aluminium armour
Figure 4
:
Polymeric armour and continuous corrugated Al armour - 3/C 350 kcm 15 KV- after 250 joules of Impact
▲
Figure 5
:
Illustrated damage on insulation shield
with copper tape removed
▲
Figure 6
:
Polymeric armour and continuous
corrugated Al armour – 9/C #12 AWG 600 V cables –
after 80 joules of impact and overall jacket removed
▲
Figure 7
:
9/C #12 AWG insulated core removed
from polymeric armour design after 80 Joules
impact
▲
Figure 8
:
9/C #12 AWG insulated core removed
from continuous corrugated Al armour after
80 Joules impact
▲